Doppler radar systems

ABSTRACT

Embodiments of the invention are concerned with improvements to Doppler radar systems, and are suitable for use with frequency scanning radar systems. 
     In one arrangement the improvements are embodied in a frequency scanning radar controller for use in controlling a frequency generator; the frequency generator is arranged to generate a plurality of sets of signals, each set of signals having a different characteristic frequency and comprising a plurality of signals transmitted at a selected rate, and the radar controller is arranged to select the rate in dependence on the characteristic frequency. 
     The invention can be embodied in a pulsed radar, for which each set of signals comprises a set of pulsed signals and each pulsed signal is repeated at a selected repetition rate. The radar controller is then arranged to modify the repetition rate in dependence on the characteristic frequency of the pulsed signal. The invention can alternatively be embodied in a frequency modulated radar system, for which each set of signals comprises a sequence of modulation patterns. The radar controller is then arranged to modify a given modulation pattern in dependence on the characteristic frequency of the signal being modulated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of International Patent ApplicationNo. PCT/EP2006/068731 filed on Nov. 21, 2006 and entitled “IMPROVEMENTSTO DOPPLER RADAR SYSTEMS”, the contents and teachings of which arehereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to improvements to Doppler radar systems,and is particularly, but not exclusively, suitable for use withfrequency scanning radar systems.

BACKGROUND OF THE INVENTION

Radar systems are used to detect the presence of objects and to measurethe location and movement of objects. It is common for such radarsystems to sweep across a given region, scanning the region for thepresence of such objects. In order to sweep over the region the radarsystems either employ mechanical devices comprising an antenna thatphysically moves in space, or electronic devices comprising elementsthat are arranged to steer radiation as it is transmitted and/orreceived. One known group of such electronic devices is frequencyscanning arrays, which, in response to input signals of varyingfrequencies, can steer a beam in an angular plane. Radar systems canalso employ frequency scanning arrays for detection avoidance and/or inthe presence of other radar systems and/or to counteract frequencyjamming equipment (e.g. by hopping between operating frequencies inorder to avoid detection of, interference with, or jamming of, the radarsystem).

Radar systems are commonly used to identify the Doppler frequency oftargets so as to identify the magnitude and direction of movementthereof. In view of the fact that a target's Doppler frequency isdependent on the radar's carrier frequency, the use of frequencyscanning antennas in a radar system presents problems in relation to theprocessing and interpretation of received signals.

SUMMARY OF THE INVENTION

A particular problem encountered when employing frequency scanningantennas is that the inherent and required variation in frequencies willmodify the Doppler frequencies for a given target, thus modifying theinferred motion of the target and complicating any velocity ambiguityresolution for targets whose Doppler frequencies fall close to half ofthe sampling frequency (e.g. the pulse repetition frequency of theradar). The inventors realised that by varying the period of thefrequency sweeps in proportion to the carrier frequency, the normalisedDoppler frequency remains substantially constant.

Accordingly in relation to an aspect of the present invention, theinventors have developed a frequency scanning radar controller for usein controlling a frequency generator, the frequency generator beingarranged to generate a plurality of sets of signals, each set of signalshaving a different characteristic frequency and comprising a pluralityof signals transmitted at a selected rate, wherein the radar controlleris arranged to select the rate in dependence on the characteristicfrequency.

The invention can be embodied in a pulsed radar, for which each set ofsignals comprises a set of pulsed signals and each pulsed signal isrepeated at a selected repetition rate. The radar controller is thenarranged to modify the repetition rate in dependence on thecharacteristic frequency of the pulsed signal.

The invention can alternatively be embodied in a frequency modulatedradar system, for which each set of signals comprises a sequence ofmodulation patterns. The radar controller is then arranged to modify agiven modulation pattern in dependence on the characteristic frequencyof the signal being modulated.

It is known to modify the modulation pattern of a continuous wavesignal, but known methods either modify the signal randomly, asdescribed in US2004/0130482, or on the basis of a sequence of values, asdescribed in US2005/0184903. In either case the modulation patterns aremodified either to overcome external interference, or/and interferencebetween respective radar systems, or/and problems with over-rangereturns (these being signals received from targets that are located at adistance from the radar system such that their return signals arereceived after transmission of a next modulation pattern in thesequence). In each of these scenarios the motivation for modifying themodulation pattern is entirely unrelated to aspects of the transmittedsignal.

In the continuous wave embodiments of the invention the radar controlleris arranged to modify the duration of individual patterns in thesequence, thereby modifying the modulation pattern. In one arrangementeach modulation pattern of the sequence comprises a linear ramp periodand a dwell period, and the radar controller is arranged to modify theduration of dwell periods of respective modulation patterns in thesequence, thereby modifying the modulation pattern. In anotherarrangement each modulation pattern of the sequence comprises a linearramp period and a descent period, and the radar controller is arrangedto modify the duration of descent periods of respective modulationpatterns in the sequence, thereby modifying the modulation pattern. Inother arrangements the modulation pattern includes a combination of aramp period, a descent period and a dwell period, in which case theduration of either of the descent or dwell periods can be modified.

The frequency scanning radar controller interoperates with a frequencygenerator that is arranged to output frequency modulated signals basedon a range of characteristic frequencies. Conveniently the frequencygenerator is responsive to inputs from the controller—indicative of therespective durations—so as to modulate the characteristic frequency.

According to a further aspect of the present invention there is providedan audio system for outputting audio data derived from signals receivedby a frequency scanning radar system, the frequency scanning radarsystem comprising a signal processor arranged to derive tone data fromsaid received signals and a frequency scanning radar controlleraccording to any one of the preceding claims, wherein the audio systemis arranged to playback the derived tone data at a constant rate.

Preferably the constant rate is selectable in dependence on saidduration of individual modulation patterns of the sequence and eachcycle of audio data comprises data derived from one or more saidmodulation patterns in the sequence. The data derived from one saidmodulation pattern in the sequence is played back at least once in eachcycle of audio data, and, in the case where the audio rate is greaterthan that corresponding to the duration of individual modulationpatterns, the data derived from one said modulation pattern in thesequence is played back more than once in each cycle of audio data.

Conveniently the audio system is configured so as to control transitionsbetween consecutive cycles of audio data—e.g. by means of an audiofading function that can be applied to the audio data appearing atrespective ends of a given cycle of audio data.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing components of a radar systemaccording to embodiments of the invention;

FIG. 2 is a schematic block diagram showing an arrangement of componentsof a frequency generator shown in FIG. 1;

FIG. 3 is a schematic diagram showing a modulation pattern for use bythe frequency generator of FIG. 2;

FIG. 4 is a schematic block diagram showing an alternative arrangementof components of a frequency generator shown in FIG. 1;

FIG. 5 is a schematic flow diagram showing steps performed by thecontroller shown in FIG. 1 during scanning of the radar system of FIG.1; and

FIG. 6 is a schematic diagram showing processing of signals in relationto a transmitted modulation pattern.

Several parts and components of the invention appear in more than oneFigure; for the sake of clarity the same reference numeral will be usedto refer to the same part and component in all of the Figures. Inaddition, certain parts are referenced by means of a number and one ormore suffixes, indicating that the part comprises a sequence of elements(each suffix indicating an individual element in the sequence). Forclarity, when there is a reference to the sequence per se the suffix isomitted, but when there is a reference to individual elements within thesequence the suffix is included.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a radar system 1 with which embodiments of the inventionoperate, the radar system 1 comprising a power source 10, a controller12, and a computer 14, the power source and computer 10, 14 beingarranged to provide power to, and operational control over, thecontroller 12. The controller 12 comprises a microprocessor and a set ofinstructions (not shown) for execution thereby, effectively generatingcontrol signals that cause the RF frequency source, or signal generator16, to output RF energy at a specified frequency F_(OUT), and thisoutput signal, under control of amplifiers 20, drives antenna 22. Aswill be described in more detail below, the RF frequency source 16generates signals within a range of frequencies, causing the antenna 22to transmit beams in different angular directions, thereby scanning overa region beyond the radar system 1.

The radar system 1 also includes a receiving antenna 32, which receivesradiated signals reflected back from objects, and passes the receivedradiation through amplifier components 20′ to mixer 34. The mixer 34comprises two inputs: a first connected to the RF source 16; and asecond connected to the receiving antenna 32. The output of the mixer 34is fed to an Analogue to Digital converter ADC 36, to produce adigitised signal for input to the signal processor 38, which performsanalysis of the received signal. The signal processor 38 performs aspectral analysis on the received signals, because the range between theradar system and external (reflecting) objects is contained as frequencyinformation in the signal. Aspects of the receiving and processingcomponents are described in detail below, but first aspects of the RFfrequency source and antenna will be described.

FIG. 2 shows components of a RF frequency generator that can be employedto generate signals having a range of frequencies. Referring to FIG. 2,the frequency generator 16 comprises a frequency source 200, firstcircuit portion 210 and a second circuit portion 220. The first circuitportion 210 comprises a frequency divider 205, a phase comparator 209, afilter 211, and a Voltage Controlled Oscillator VCO 213, while thesecond circuit portion 220 comprises a frequency divider 207, staticmultiplier 201 and a mixer 203. The mixer 203 receives, as input,signals output from the VCO 213 and signals from the high grade, staticmultiplier 201, and generates signals of frequency equal to thedifference between the frequencies of the two inputs (f₃). The valuesR1, R2 characterising the frequency dividers 205, 207 are selectable,and the phase comparator 209 is arranged to compare the frequency andphase of signals output from the frequency dividers 205, 207 (f₃/R2 andf_(ref)), so as to output a phase-error signal, of magnitude dependenton the difference between f₃/R2 and f_(ref). The phase-error signal isinput to the VCO 213, and the first circuit portion 210 operates so asto cause the output from the VCO 213 to stabilise in dependence on thephase-error signal. Thus different values of R2 can be used to force theloop to stabilise at a frequency multiple of the input signal. In onearrangement the frequency source 200 is embodied as a crystal oscillatorand in another arrangement as a SAW oscillator.

The signals output from the second circuit portion are then modulated byoutput f_(DDS) of a third circuit portion 230, which in one arrangementcomprises a Direct Digital Synthesiser 223, a Digital to AnalogueConverter DAC 225 and a low pass filter 227. The third circuit portion230 is configured, under control of the controller 12 shown in FIG. 1,to generate a repeating pattern comprising a linear frequency ramp. Theramp has a specified duration and magnitude, values of which areprogrammed via the controller 12. FIG. 3 shows an example of one suchfrequency ramp 301 ₁ for a given carrier frequency f_(c1), the durationof which is approximately 64 μs, the magnitude of which, in terms ofrange of frequencies (f_(DDS,max)-f_(DDS,min)), is approximately 20 MHz,and is followed by a flyback ramp 303 ₁ to prepare the third circuitportion 230 for the next ramp 301 ₂. The pattern repeats at apredetermined rate—in the present example a rate of 8 KHz (thus a sweeprepeat period 307 of 125 μs (subject to the modifications describedlater in the specification)) is a convenient choice. Such a modulationpattern is entirely conventional and the foregoing details are includedas illustrative; the skilled person will appreciate that any suitablevalues could be selected, dependent upon the use of the radar system(e.g. the nature of the targets to be detected). For each carrierfrequency, the third circuit portion 230 is arranged to repeat thelinear ramp pattern a specified number of times, e.g. 256 or 512 times,the number being dependent on the desired signal to noise ratio andtherefore a design choice. Whilst the third circuit portion 230 shown inFIG. 2 comprises digital synthesiser components, it could alternativelybe embodied using analogue components such as a sawtooth generator andVCO or similar. Preferably, and in order to save power, it is to benoted that the antenna 22 is not energised during either of the flybackramp or dwell periods 303, 305.

Turning back to FIG. 2, the output f_(DDS) of the third circuit portion230 is input to a fourth circuit portion 240, which comprises a phasecomparator 233, a filter 235, a Voltage Controlled Oscillator 237 and amixer 231. The mixer receives signals output from the second circuit(having frequency f₂) and signals output from the VCO 237 (havingfrequency f₅) and outputs a signal at a frequency equal to thedifference in frequency between f₂ and f₅. The phase comparator 233outputs a phase-error signal, of magnitude dependent on the differencebetween (f₂-f₅) and f_(DDS) to the VCO 237, and the fourth circuitportion 240 operates so as to cause the output from the VCO 237 tostabilise accordingly.

The signals output from the fourth circuit portion 240 (having frequencyf₅) are then combined, by means of mixer 241, with signals of areference frequency f₄, which are signals output from the oscillator 200having been multiplied by a second static multiplier 251, and the outputis filtered (filter 243) so as to generate a signal having an outputfrequency F_(OUT). It will be appreciated from FIG. 2 that when thesignal generator 16 is operable to output signals corresponding to acarrier frequency of between 15.5 GHz and 17.5 GHz, for a crystaloscillator 200 outputting signals of frequency 100 MHz, the secondstatic multiplier 251 is of the order 130. Thus for each carrierfrequency the frequency generator 16 generates a repeating pattern offrequency modulated signals of various carrier frequencies.

Whilst the signal generator 16 could be used to generate frequencieswithin any selected range of frequencies, when used as a ground-basedradar system, the frequency range can fall within the X band (8 GHz-12.4GHz); the Ku band (12.4 GHz-18 GHz); the K band (18 GHz-26.5 GHz); orthe Ka band (26.5 GHz-40 GHz), and most preferably within the Ku band,or a portion within one of the afore-mentioned bands.

Whilst in preferred arrangements the first and second circuit portions210, 220 of frequency generator 16 are embodied as shown in FIG. 2, thefrequency generator 16 could alternatively be based on an arrangementcomprising a plurality of fixed frequency oscillators, as shown in FIG.4, one of which is selected via switch 400 so as to generate a signal atfrequency f₂. Judicial selection of an appropriate fixed frequencyoscillator (e.g. a crystal oscillator) means that the frequencygenerator 16 can incur minimal phase noise, since the signals are takendirectly from one of the oscillators. However, this advantage isaccompanied by a corresponding limitation, namely that there is no meansfor fine-tune adjustment of the carrier frequency, which can be adisadvantage when working with antennas 22 that require fine tuning ofthe carrier frequency to achieve optimal beamwidth distribution (interms of distribution of radiation within the lobes).

It will be appreciated from the foregoing that the frequency f_(OUT) ofsignals output from the signal generator 16 is controlled by thecontroller 12. In addition to controlling the duration and rate of theramp as described above, the controller 12 is arranged to select adifferent value for carrier frequency after the ramp pattern has beenrepeated a specified number of times for a given carrier frequency(examples of 256 and 512 were given above). In one arrangement thevalues for the carrier frequency can be selected from a look-up tableaccessible to the controller 12 (e.g. stored in local memory or on thecomputer 14), this look-up table being particular to a given antenna 22,32.

Operation of the radar system 1 described above will now be describedwith reference to FIG. 5, which is a schematic flow diagram showingsteps carried out by the controller 12. At step S5.1 the controller 12energises the antenna 22; at S5.3 the controller 12 retrieves the valueof the first carrier frequency f_(c1) (e.g. from the look-up tablementioned above), and at step S5.5 the controller 12 sets the values ofR1 and R2 accordingly (to set the carrier frequency). Subsequently thethird circuit portion 230 is triggered to generate the ramp pattern apredetermined number of times Rmp_(max) (S5.7), so as to repeatedlymodulate the carrier frequency. Having reached Rmp_(max), the controllerretrieves the value of the next carrier frequency f_(c2) and sets thevalues R1, R2. Preferably the overall duration of step S5.7—in otherwords the duration of any given set of repetitions of the linear ramp301 _(i) pattern—is the same for all values of the carrier frequency,f_(cj).

The description has thus far focussed on the generation and transmissionof signals from the radar system 1; referring to FIG. 1, aspects ofreceiving and processing of signals will now be described. As describedabove the radar system 1 preferably also includes a separate antenna 32for receiving radiation. The signals received by the receiving antenna32 are input to mixer 34, together with the output F_(OUT) from the RFfrequency generator 16, and, in accordance with standard homodyneoperation, the output from the mixer 34 is fed through an ADC 36 toproduce a digitised Intermediate Frequency (F_(if)) signal as input tothe signal processor 38. Energising of the receiving antenna 32 isperformed under control of the controller 12, and, as for thetransmitting antenna 22, this occurs during the linear ramp period only301 _(i).

The signal processor 38 is conveniently embodied as a programmable logiccontroller (PLC) and a plurality of software components, which runlocally on the PLC 38 in response to signals received from aconventional PC computer 14 and which are written using the proprietaryprogramming language associated with the PLC 38.

As described above, the radar system 1 operates according to homodyneprinciples, which means that the Intermediate Frequency F_(if) is equalto differences between the received signal frequency and the transmittedsignal frequency. In embodiments of the invention, as will beappreciated from the foregoing and FIGS. 2 and 3 in particular, theoutput of the radar system 1 is a sequence of frequency sweeps 301 _(i).It is a well known principle of radar that targets located in the pathof a given transmitted beam will reflect the transmitted signals; sincethe transmitted signal in embodiments of the present invention comprisesa linear frequency sweep 301 _(i), the reflected signals also comprise alinear frequency sweep. Targets that are stationary will generatereflected signals that are identical to the transmitted signals (albeitsomewhat attenuated), but separated therefrom at a constant frequencydifference referred to herein as a tone. Referring to FIG. 6, it will beappreciated from the Figure that different targets T1, T2—located atdifferent distances from the radar system 1—reflect the transmittedsweep 301 _(i) at different delays in relation to the time oftransmission, and that therefore targets T1, T2 at these differentlocations will be associated with different tones Δf₁, Δf₂.

In view of the fact that the signals output from the mixer 34 containtones, the signal processor 38 is arranged to delay the processing ofsignals until the ramp 301 has traveled to the extents of the detectionregion and back. Thus for example, if the detection region extended to4.5 km from the radar system 1, the signal processor 38 would startprocessing signals output from the mixer 34 at:

$\frac{4500 \times 2}{3 \times 10^{8}} = {30\mspace{14mu} µ\; s\mspace{14mu} {from}\mspace{14mu} {the}\mspace{14mu} {start}\mspace{14mu} \text{of}\mspace{14mu} {transmission}\mspace{14mu} {of}\mspace{14mu} a\mspace{14mu} {given}\mspace{14mu} {ramp}\mspace{14mu} {301_{i}.}}$

Considering, for the sake of clarity, one processing period 601 ₁, thesignal processor 38 essentially calculates the Doppler frequency oftargets within range of the transmitted beam—and which reflect thetransmitted beam. This is achieved by sampling the received tones Δf₁,Δf₂ . . . Δf_(m) at a predetermined sampling rate. The sampling rate isselected so as to as ensure that phase shifts of the transmitted signal,which are induced by moving targets, can be captured. The skilled personwill appreciate that this is dependent on the ramp rate, since theDoppler frequency is dependent on the frequency of the transmittedsignal:

Doppler Frequency=2vf _(c) /c  [Equation 1]

Thus, the output of the ADC 36 falling within the processing period 601₁ will be processed a predetermined number of times (corresponding tothe sampling rate) by the signal processor 38. Each sample will containzero, one or a plurality of tones, each relating to signals reflectedfrom targets.

As will be appreciated from the foregoing, the linear ramp 301 _(i) istransmitted a plurality of times for each carrier frequency. Accordinglythe signal processor 38 processes data received during a correspondingplurality of processing periods 601 _(i), and generates, by means of aRange FFT, a set of return samples, individual members of which areassigned to a respective set of range gates for each said processingperiod 601 ₁. Thus the output of the Range FFT, for a given processingperiod 601 ₁, is frequency information distributed over so-called rangegates. As is well known in the art, range gates represent successivedistances from the radar system 1, such that if return samples fallwithin a given range gate, this indicates the presence of a targetlocated at a distance equal to the range gate within which the returnsample falls.

Having transformed the received signals into range gates the signalprocessor 38 is arranged to take the FFT of the return samples assignedto each given range gate. In the current example it will be appreciatedthat each set of range gates corresponds to transmission of a linearramp 301 _(i) (for a given carrier frequency), and that the samplingrate in relation to range gates—the rate at which data falling within agiven range gate are measured—is the frequency at which the pattern oftransmission of linear ramps 301 _(i) is repeated (commonly referred toas the Pulse Repetition Frequency (PRF)). In the example given above,and with reference to FIG. 3, this is nominally 8 KHz. Accordingly, foreach carrier frequency, the signal processor 38 effectively generates anarray of data, each row in the array corresponding to a given processingperiod 601 _(i), and each column in the array corresponding to a givenrange gate.

The FFT output comprises amplitudes and phases of various components ofsignal energy which fall on frequencies spaced linearly at the inverseof the duration of a complete signal sample set (in embodiments of theinvention, the signal set comprises tones, not absolute frequencyvalues). In the current example, therefore, and assuming the signalsample set for a given carrier frequency to comprise the 512 linearramps 301 ₁ . . . 301 ₅₁₂ transmitted at a rate of 8 KHz, there are 512FFT output bins spaced at a Doppler frequency of 8000/512=15.625 Hz; fora carrier frequency of 15 GHz, this is equivalent to 0.15625 m/s. Thuseach FFT output bin represents a different velocity; stationary targetswill appear in bin 0, while moving targets will appear in a bindependent on their velocity (a target travelling at 10 m/s will appearin bin 64).

As is known in the art, the signal processor 38 can be arranged to storeeach set of range gate samples in a “row” of a conceptuallyrectangularly-organised memory, referred to as a corner store, each rowcorresponding to range gates falling within a given processing periods601 _(i) and thus to a particular linear ramp 301 _(i). Once all 512linear ramps 301 ₁ . . . 301 ₅₁₂ have been transmitted, each column—i.e.each range gate—is read out and input to a FFT for processing thereby inthe manner described above.

From Equation 1, it will be appreciated that the Doppler frequency isdirectly proportional to the carrier frequency f_(c). Therefore when thecarrier frequency varies—as is the case with frequency scanningantennas—the variation in carrier frequency will modify the derivedDoppler frequencies so as to effectively scale the magnitude of thefrequencies. For example, a radar system that operates between 15.5 GHzto 17.5 GHz can generate Doppler frequencies, for a given target, whichvary by ±6%. This equates to a system-generated shift in Dopplerfrequency of more than 2 semitones, and a variation in ambiguous Dopplervelocity from 70 mph to 79 mph, which can complicate the task ofremoving velocity ambiguity from targets moving at these speeds andabove.

Accordingly the controller 12 is arranged to modify the sweep repeatperiod 307 (referred to as Pulse Repetition Frequency (PRF) or sweeprepetition frequency) such that the sweep repetition frequency isproportional to the carrier frequency, thereby effectively removing thissystematic aberration. Turning back to FIG. 5, this means that step S5.5performed by the controller 12 in relation to carrier frequency f_(cj)retrieved at step S5.3 is accompanied by calculation of a sweep period307 for the particular value of this carrier frequency f_(cj). Inpreferred embodiments of the invention the linear sweep period 301remains unchanged (so that the effect of this adjustment does not affectthe processing performed by the signal processor 38), and the controller12 adjusts the duration of the flyback and/or dwell periods 303, 305;most preferably the dwell period 305 is modified. Of course all of therepetitions of the sweep repeat period 307 ₁,f_(cj) . . . 307 ₅₁₂,f_(cj)are identical for a given carrier frequency f_(cj) (step S5.7). In oneembodiment the controller 12 has access to a look-up table, which listssweep repeat periods 307 _(j) for discrete carrier frequencies f_(cj).Conveniently such data could be stored in the look-up table that isaccessed by the controller at step S5.3, when identifying a next carrierfrequency f_(cj).

As described above in relation to FIG. 5, the overall duration D of stepS5.7 is preferably maintained constant. When, as is the case withembodiments of the invention, the sweep repeat period 307 _(j) varies inaccordance with carrier frequency f_(cj), the duration of 512repetitions applied in respect of each different carrier frequencyvaries; thus, of itself, the period associated with 512 repetitionswould not be of duration D for all carrier frequencies. In order toensure that the duration is nevertheless constant, the controller 12 isconfigured to wait for a period equal to the time difference between theend of 512 repetitions and duration D before moving onto the nextinstance of steps S5.3, S5.5 and S5.7 (i.e. for a different carrierfrequency). In the present example the value of D is preferably set tothe sum of 512 sweep repeat periods 307 corresponding to the duration ofthe longest sweep repeat period (and thus that associated with thelowest carrier frequency f_(cj)).

This feature of the controller 12 is advantageous for configurations inwhich the linear ramp period 301 is constant (in FIG. 3 it is shown as64 μs), incurring a fixed transmitter power dissipation: maintainingduration D for the overall duration of step S5.7 means that the averagetransmitter dissipation is constant and independent of variations to thesweep repeat period 307 (PRF). As a result the temperature of thetransmitter T_(x) is maintained at a constant level, which, in turn,minimises the variations in parameters that are temperature dependent.

Preferably the Doppler frequencies are scaled and output as tones withinthe audible range and at a fixed audio sample rate. Playing back thetones at a fixed rate is a convenient approach in view of the fact thatthe Doppler frequencies have been normalised in relation to thevariation in carrier frequency.

As an alternative to selecting sweep repeat periods 307 _(j) as afunction of carrier frequency f_(cj), the sweep repeat period 307 _(j)could be varied incrementally, for example linearly, based on theapproximation 1+α≈1/(1−α) for α<<1. For the example frequency range of15.5 GHz-17.5 GHz, the sweep repeat period 307 for a carrier frequencyof 15.5 GHz could be 140.65 μs, and period 307 for a carrier frequencyof 17.5 GHz could be 125 μs, while the sweep repeat period 307 forcarrier frequencies between the extents of this range can be selected soas to vary linearly between 125 μs and 140.65 μs. As for the firstalternative—where the sweep repeat period 307 is varied discretely asthe carrier frequency varies—the linear ramp 301 and thus the processingperiods 601 remain unchanged for all values of the sweep repeat period307. The net change in Doppler frequency is then reduced to ±0.2% andthe ambiguous Doppler velocity varies from 78.7 mph to 79.0 mph. As forarrangements for which the sweep repeat period 307 (PRF) is selected indirect dependence on the carrier frequency, the duration of any givenset of repetitions of sweep repeat period can be fixed at duration D.

As described above, a radar system according to embodiments of theinvention can conveniently be used for transceiving radio frequencyenergy and processing the same so as to output an audible representationof Doppler frequencies and thus identifying moving targets. The signalprocessor 38 is arranged to transmit data indicative of the Dopplerfrequencies to the computer 14, which comprises a suite of softwarecomponents 39 arranged to convert the Doppler frequencies to audiblesignals and to playback the same. As described above, the DopplerFrequencies are normalised by processing the received signals at avariable rate, the rate being selected in dependence on the carrierfrequency of the transceived signal, while the rate at which the audiois played back is substantially constant. Preferably the post processingsoftware components 39 are arranged to ensure smooth transition betweenrespective audio bursts by controlling the playback rate in relation tothe rate at which, for a given range gate, data have been processed bythe signal processor 38 (i.e. the frequency at which the pattern oftransmission of linear ramps 301 _(i) is repeated).

If the PRF is varied between 7 KHz and 8 KHz and the audio playback rateis 8.5 KHz, then in the absence of suitable phased-audio control, therewill be gaps in the audio output, which presents an interruption to anyaudible analysis of the Doppler data; one way of mitigating this is torecycle Doppler data during periods that would otherwise be silent,until such time as further Doppler data are made available from thesignal processor 38. In order to ensure a smooth transition betweenrespective sets of Doppler data, the computer 14 would be arranged tofade-out previous, and fade-in current, sets of Doppler data. As analternative, the audio playback rate could be set at a value lower thanthe PRF (e.g. for the current example, 6.9 KHz) so that respective setsof Doppler data overlap; the periods of overlap can be managed usingappropriately selected fade-in and fade-out functions. As a furtheralternative, previous sets of Doppler data could be played back by thecomputer 14 during what would otherwise be gaps in the audio output.This alternative is particularly appropriate for arrangements in whichthe radar is transmitting in a fixed direction and is thereforeoperating at a constant carrier frequency, meaning that the audio burstsare coherently related to one another.

In arrangements where the duration of sets of repetitions of the linearramp period 301 _(i) is constant (duration D), any set of Doppler data(corresponding to a given carrier frequency f_(cj)) will arrive at thesignal processor 38 a constant rate, which means that the softwarecomponents 39 can be configured to apply the same conditions in relationto overlaps and/or gaps in the Doppler data (since the amount of overlapor gap can always be calculated from duration D). An advantage of thisarrangement is that it simplifies the logic associated with thepost-processing software components 39 and enables more constant audiooutput over the varying PRF.

A particular feature of a radar system according to embodiments of theinvention is that the software components 39 are arranged to transmitdata output from the signal processor 38 to a remote processing system,for tracking and monitoring of targets. Most preferably the softwarecomponents 39 are arranged to transmit data output from the signalprocessor 38 each time the carrier frequency—and thus region beingscanned—changes. This means that the computer 14 acts primarily as aconduit for data, while the data intensive processes of correlatingtargets between successive scans, rendering of targets upon a displayand prediction of target behaviour can be performed by a separateprocessing system. In a preferred arrangement the data are transmittedwirelessly, but it will be appreciated that any suitable transmissionmeans could be used.

Additional Details and Alternatives

Whilst in the foregoing the linear ramp 301 is independent of variationsin the sweep repeat period, the controller 12 could alternatively modifythe duration and/or slope of the linear ramp. Whilst this is not apreferred method, because operation of the signal processor 38 (inparticular in relation to the processing periods 601) would have to bemodified, modifying the slope is a convenient method when more than oneradar system is being utilised in a given region, since the differencein slopes of the linear ramp can be used to distinguish between outputfrom respective radar systems.

In relation to the form of the frequency scanning antenna, the antenna22, 32 can be embodied as a travelling wave antenna or as a waveguide inthe form of a serpentine antenna or similar. Suitable antennas aredescribed in US patents U.S. Pat. No. 4,868,574 and applicant'sco-pending application entitled “Frequency scanning antenna”.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

1. A frequency scanning radar controller for use in controlling afrequency generator, the frequency generator being arranged to generatea plurality of sets of signals, each set of signals having a differentcarrier frequency and comprising a sequence of patterns, at a selectedrate, wherein the radar controller is arranged to select the rate independence on the carrier frequency.
 2. (canceled)
 3. A frequencyscanning radar controller according to claim 1, wherein the radarcontroller is arranged to modify a given modulation pattern independence on the carrier frequency of the signal being modulated.
 4. Afrequency scanning radar controller according to claim 3, wherein theradar controller is arranged to modify the duration of individualmodulation patterns of the sequence, thereby modifying the modulationpattern.
 5. A frequency scanning radar controller according to claim 3,wherein each modulation pattern of the sequence comprises a ramp periodand an intervening period, and the radar controller is arranged tomodify the duration of the intervening periods of respective modulationpatterns in the sequence, thereby modifying the modulation pattern.
 6. Afrequency scanning radar controller according to claim 5, wherein eachmodulation pattern of the sequence comprises a linear ramp period and adwell period, and the radar controller is arranged to modify theduration of dwell periods of respective modulation patterns in thesequence, thereby modifying the modulation pattern.
 7. A frequencyscanning radar controller according to claim 5, wherein each modulationpattern of the sequence comprises a linear ramp period and a descentperiod, and the radar controller is arranged to modify the duration ofdescent periods of respective modulation patterns in the sequence,thereby modifying the modulation pattern.
 8. A frequency scanning radarcontroller according to claim 3, wherein each modulation pattern of thesequence comprises a linear ramp period.
 9. An audio system foroutputting audio data derived from signals received by a frequencyscanning radar system, the frequency scanning radar system comprising asignal processor arranged to derive tone data from said received signalsand a frequency scanning radar controller according to claim 1, whereinthe signal processor is arranged to process the received signals at arate dependent on the rate of transmission of the sequence of modulationpatterns and the audio system is arranged to playback the derived tonedata at a constant rate.
 10. An audio system according to claim 9,wherein the constant rate is selectable in dependence on said durationof individual modulation patterns of the sequence.
 11. An audio systemaccording to claim 9, wherein each cycle of audio data comprises dataderived from one or more said modulation patterns in the sequence. 12.An audio system according to claim 9, wherein the data derived from onesaid modulation pattern in the sequence is played back at least once ineach cycle of audio data.
 13. An audio system according to claim 7,wherein at least some data derived from one said modulation pattern inthe sequence is played back more than once in each cycle of audio data.14. An audio system according to claim 9, wherein the audio system isarranged to control transitions between consecutive cycles of audiodata.
 15. An audio system according to claim 14, wherein the audiosystem is arranged to apply an audio fading function to tone data playedback at respective ends of a given cycle of audio data.
 16. An audiosystem according to claim 14, wherein the audio system is arranged toselect tone data from a previous cycle of audio data for use in playbackprior to receipt of a next cycle of audio data.
 17. An audio systemaccording to claim 16, wherein the tone data is selected from an end ofthe previous cycle of audio data.
 18. An audio system according to claim9, wherein the constant rate is selected so as to be greater than a ratecorresponding to said duration of individual modulation patterns of thesequence.
 19. A scanning radar system comprising a frequency generator,a frequency scanning antenna and a receiver, wherein the frequencygenerator is arranged to generate a plurality of sets of signals, eachset of signals having a different carrier frequency and comprising aplurality of swept signals transmitted at a selected rate, the frequencyscanning antenna being arranged to cooperate with the frequencygenerator so as to transceive radiation over a region having an angularextent dependent on the said carrier frequencies, wherein the receiveris arranged to process signals received from a target in dependence onthe selected rate so as to alleviate variations in Doppler frequencyarising from transceiving of the radiation over the angular extent. 20.A frequency scanning radar controller for use in controlling frequencymodulation of a continuous wave signal, the continuous wave signalhaving a carrier frequency and being modulated by a sequence ofmodulation patterns, wherein the radar controller is arranged to modifya given modulation pattern in dependence on the carrier frequency of thesignal being modulated